Unsubscribing from an Event in .NET

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Unsubscribing from an Event
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Knowing that you use the += operator to attach a delegate to an event, you can probably guess that you use the = operator to detach a delegate from an event. Calling the = operator removes the method from the event s internal delegate collection. This action is often referred to as unsubscribing from the event.
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Interrupting Program Flow and Handling Events
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Raising an Event
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An event can be raised, just like a delegate, by calling it like a method. When you raise an event, all the attached delegates are called in sequence. For example, here s the TemperatureMonitor class with a private Notify method that raises the MachineOverheating event:
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class TemperatureMonitor { public delegate void StopMachineryDelegate; public event StopMachineryDelegate MachineOverheating; ... private void Notify() { if (this.MachineOverheating != null) { this.MachineOverheating(); } } ... }
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This is a common idiom. The null check is necessary because an event eld is implicitly null and only becomes non-null when a method subscribes to it by using the += operator. If you try to raise a null event, you will get a NullReferenceException. If the delegate de ning the event expects any parameters, the appropriate arguments must be provided when you raise the event. You will see some examples of this later. Important Events have a very useful built-in security feature. A public event (such as MachineOverheating) can be raised only by methods in the class that de nes it (the TemperatureMonitor class). Any attempt to raise the method outside the class results in a compiler error.
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Understanding WPF User Interface Events
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As mentioned earlier, the .NET Framework classes and controls used for building graphical user interfaces (GUIs) employ events extensively. You ll see and use GUI events on many occasions in the second half of this book. For example, the WPF Button class derives from the ButtonBase class, inheriting a public event called Click of type RoutedEventHandler. The RoutedEventHandler delegate expects two parameters: a reference to the object that caused the event to be raised and a RoutedEventArgs object that contains additional information about the event:
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public delegate void RoutedEventHandler(Object sender, RoutedEventArgs e);
Part III
Creating Components
The Button class looks like this:
public class ButtonBase: ... { public event RoutedEventHandler Click; ... } public class Button: ButtonBase { ... }
The Button class automatically raises the Click event when you click the button on-screen. (How this actually happens is beyond the scope of this book.) This arrangement makes it easy to create a delegate for a chosen method and attach that delegate to the required event. The following example shows the code for a WPF form that contains a button called okay and the code to connect the Click event of the okay button to the okayClick method:
public partial class Example : System.Windows.Window, System.Windows.Markup. IComponentConnector { internal System.Windows.Controls.Button okay; ... void System.Windows.Markup.IComponentConnector.Connect(...) { ... this.okay.Click += new System.Windows.RoutedEventHandler(this.okayClick); ... } ... }
This code is usually hidden from you. When you use the Design View window in Visual Studio 2008 and set the Click property of the okay button to okayClick in the Extensible Application Markup Language (XAML) description of the form, Visual Studio 2008 generates this code for you. All you have to do is write your application logic in the event handling method, okayClick, in the part of the code that you do have access to, in the Example.xaml.cs le in this case:
public partial class Example : System.Windows.Window { ... private void okayClick(object sender, RoutedEventArgs args) { // your code to handle the Click event } }
The events that the various GUI controls generate always follow the same pattern. The events are of a delegate type whose signature has a void return type and two arguments. The rst
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